Sensors are key feedback devices on many electromechanical systems. There is a wide variety of sensors available and new sensor technologies are continuously being developed. One of the most common position sensors utilized to measure the moving parts within a mechanical system is the optical encoder. An optical encoder is a feedback device that converts motion or positional information into digital signals. Optical encoders produce a digital output based on an encoded media that passes either through or by the optical encoder. In general, the media is encoded with alternating light and dark regions (or slots) on the surface of the media.
The light and dark regions may contain opaque and transparent segments, respectively, that interrupt a light beam between a light source and a detector in the optical detector. The optical encoder output is then either a binary “ON” or “OFF,” depending on whether the optical encoder is over a light or dark region on the media. The electronic signals generated by the optical encoder are then passed to a controller that is capable of determining the position and velocity of the detector based upon the received signals.
In FIG. 1, a side cross-sectional view of a typical transmissive optical encoder 100 in combination with a media codestrip 102 is shown. The optical encoder 100 may include a read-head 104, where the read-head 104 may include an emitter module 106, and a detector module 108. The read-head 104 and the codestrip 102 may move freely relative to each other in either a linear or rotational manner.
Both the emitter module 106 and detector module 108 may include optics capable of emitting and detecting optical radiation 110 from the emitter module 106 to the detector module 108. The optical radiation 110 may be visible, infrared, and/or ultraviolet light radiation. The emitter module 106 may include a light source (not shown) such as a light emitting diode (“LED”) and the detector module 108 may include an array of photo-detectors (not shown) such as photo-diodes.
Optical encoders 100 are either linear optical encoders or rotational optical encoders. Linear optical encoders may determine the velocity, acceleration and position of a read-head relative to a linear codestrip utilizing a linear scale, while rotational optical encoders may determine the tangential velocity, acceleration and angular position of a read-head relative to a circular codestrip utilizing a circular scale. However, in general there are two types of optical encoders for both linear optical encoders and rotational optical encoders.
These two types of optical encoders are known as absolute optical encoders and relative (also known as “incremental”) optical encoders. Absolute optical encoders utilize several sensors in parallel to produce bit patterns that determine the position of the read-head 104 along with its velocity and acceleration relative to the codestrip 102. Incremental optical encoders, however, only determine the velocity and acceleration of the read-head 104 but not its position relative to the codestrip 102. Incremental optical encoders are less expensive than absolute optical encoders.
In FIG. 2, a top-view of a typical transmissive linear media utilized as a codestrip 200 by an incremental linear optical encoder (not shown) is shown. The codestrip 200, FIG. 2, may include an alternating pattern of light bars 202 and dark bars 204. Utilizing the codestrip 200, the incremental linear optical encoder may determine the velocity and acceleration of the read-head (not shown) relative to the codestrip 200.
Unfortunately, incremental linear optical encoders utilizing the codestrip 200 are not capable of determining the position of a read-head relative to the codestrip 200. Absolute linear optical encoders have been utilized to solve this problem. In operation, known absolute linear optical encoders utilize multiple detectors and numerous segment patterns on the codestrip to produce different binary outputs for each segment pattern so that the detectors' positions are absolutely determined relative to the codestrip. However, known absolute linear optical encoders are significantly more expensive than incremental linear optical encoders because known absolute linear optical encoders utilize multiple detectors and numerous segment patterns on the codestrip. Therefore, there is a need for a cost effective absolute linear optical encoder capable of determining the detectors position relative to the codestrip without the cost associated with conventional absolute linear optical encoders.